Heretofore hand-entry constraint grids have been employed to receive hand entries for OCR classification. User visibility and photo-detection of constraint grids (and other guide artifacts) were optimized by spectrum management. That is, the color of the guide lines was selected to be visible to the user in ambient light, and to match (or mismatch) the detector response spectrum under detector illumination. For example, in xerography, light blue guide lines present on the original document do not appear on the copy. The silicon photo-detector layer on the charge transfer drum is responsive to blue light. The guide line image discharges the drum surface, and becomes indistinguishable from the white background of the document which also discharges the drum. In contrast, the silicon drum is not responsive to the dark text foreground on the document. The text retains the drum charge to form a charge image which is transferred to the copy for toning into a visible image. In silicon drum xerography, red lines do not discharge the drum, and appear in the toned image with the text foreground. application Ser. No. 08/567,763, filed on Dec. 5, 1995 by Radovan V. Krtolica and Roger D. Melen, and assigned to the present assignee; teaches a scanning system which employs spectrum management to delete hand-entry constraint grids. The input sheet contains constraint grids which do not appear in the output pixel image. The constraint grids are printed in a distinct color visible to the user, but not detectable by the scanner photo-detectors.
Other constraint grids, such as pair of parentheses "( )", provide left-right guides for receiving a dark vertical mark therebetween indicating a user selection. These parentheses type constraint grids are located on the original close to the entry mark, and are scanned by the photo-detectors. Reflected light from the grids may introduce noise into the output pixel image, unless they have a reflection wavelength sufficiently remote from the response portion of the detector spectrum.